There aren't 150 in service right now. Probably 100 based on recent counting of serial numbers and units we know about. My guess the current production rate is around a 40 a year and on the way to 50 a year. This might go up higher in the coming years, but they will need to keep optimizing and improving production line.
It's a huge deal to be able to produce J-20 this quickly already.
All this is paper talk. F-22 has 50% availability rate. It could not even fly off the ground when a big hurricane struck one of its based in NC and ended up damaging a whole bunch of them. And it's only getting most costly to maintain and less available as we step further away from its production ending date. The USAF decision to stop F-22 production is turning out to be a a huge disaster.
We'd have to see what J-31 looks like to get a better understanding of its stealth and overall capabilities. It looks to be the best approximation of F-35 you can find with the exception of having 2 engines instead of 1. The T/W ratios are about the same. The configurations are very similar. Based on the prototypes, J-31 production quality looks close to J-20 and F-22 and below that of F-35. Stealth wise, it looks very similar to F-35 in shaping from front and sides. It definitely has more issue in the rear compared to F-35, although F-35 is also less stealthy from rear than F-22. Unless they can fix the rear and improve production quality, I would expect J-31 to be less stealthy than F-35, but still very stealthy from most angles. In terms of electronics, it should be very well designed to use passive and LPI active sensors like J-20 and F-35. So from most aspect, it's competitive with F-35, but theoretically worse in stealth. Depending on what J-31 is fighting with, that may or may not overcome those differences.
Keep in mind that J-31 should still eat 4th gen aircraft for lunch. There is a huge gap between aircraft with high level of stealth and modern situation awareness/networking vs those that don't. So in a battlefield, things like numbers, availability, endurance, cap time and supporting aircraft will matter a lot. J-31 does not need to take down too many F-35s to be victorious. It just needs to out last F-35s and take out a good chunk of non-stealthy aircraft like tankers, AWACs and 4th generation aircraft. It's far easier to take out F-35 when they return to their flight deck of air base for maintenance rather than taking them out while they are still flying. So when certain people say that J-20 or J-31 aren't as stealthy/capable as F-35, that really completely over looks other aircraft in the theater. 5th generation aircraft are larger threats to other aircraft than they are too each other.
For PAF, the best case scenario is if IAF ends up purchasing Su-57 as its "5th generation" aircraft. The worst case scenario is if IAF ends up purchasing F-35. With the former, PAF will have a pretty large qualitative advantage. With the latter, PAF will need a little more help from PLAAF in ways to counter.
By the way, land version of FC-31 will never be available for a while. I'm guessing at least 5 more years before land version of FC-31 join service, so earliest export will probably be in 2029 to 2030 range.
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